Plant issues

[Sandy pikelny]

sorry if I'm not in the right discussion. I'm a newbie, I have a 75 gallon tank with 15 young tilapia. Most of my plants are self pollinating. The plants grow beautifully for a while, then problems show up. Peas grew stunted pods, others begin to wilt, or have brown edges. I add nutrients 1 or2x a month, depending.
My numbers are very low. When I had ammonia of 8 the nitrite/nitrate levels were ok. Reads are ammonia .50, nitrite/nitrates are 0, ph 6. I've added a bacteria booster etc.
I have an indoor system. I'd appreciate it if anyone has ideas.

Thanks, sandy :(

[Gilda Baise]

0 nitrates means your plants have no food. The number of plants you can grow depends on how much nitrogen can be produced from fish waste, which, according to studies by Dr. James Rakocy and the University of the Virgin Islands, depends on how much food your fish can eat in a day. 15 small tilapia probably can't support the number of plants you are growing. If I have done the math right (not necessarily so), according to Dr. Wilson Lennard's info (check out his website), 1 square meter of lettuce requires 60-100 grams of fish food entering the system daily (2.1 - 3.5 oz?, or about 3/4 to 1 cup of Little Strike floating catfish food, which is what I use); adult fish eat about 1% of their weight per day. To eat 100 grams of food, you would need 10 Kg of fish or about 22 lbs of adult fish. 

Also, Dr Lennard recommends a minimum ph of 6.5. 6 is the lowest at which the bacteria can efficiently convert the waste.

There is also a free calculator for backyard systems at Dr Lennard's site, which is very helpful. He normally works with commercial enterprises, so this is a wonderful gift to us backyarders. http://www.aquaponic.com.au/index.htm

[Alex Veidel]

You could also have some pests in your system. Pests like thrips, spider mites, and aphids can all stress out your plants and cause them to stunt or die. Also, those harmless little roly-poly bugs? They like to occasionally snack on new seedlings as well.

[Gilda Baise]

Also, Certain mineral deficiencies can cause wilting and brown edges, and/or stunted fruit; You might do some research on that. You didn't say if you have a raft or media system. Some plants don't like their roots in water all the time; they need air space for their roots. I've tried tomatoes both with and without media, and by far they do better with media. The plants look like they are thriving when they are quite young, but eventually fail. Check the roots of your peas. Healthy roots should be white.